Power/Performance Bits: June 30

Tiny, solar-powered sensors

Last week, at the Symposia on VLSI Technology and Circuits, MIT researchers presented a new power converter chip that can harvest more than 80 percent of the energy trickling into it, even at the extremely low power levels characteristic of tiny solar cells. Previous ultralow-power converters that used the same approach had efficiencies of only 40 or 50 percent.

Moreover, the researchers’ chip achieves those efficiency improvements while assuming additional responsibilities. Where most of its ultralow-power predecessors could use a solar cell to either charge a battery or directly power a device, this new chip can do both, and it can power the device directly from the battery.

The MIT researchers’ prototype for a chip measuring 3 millimeters by 3 millimeters. The magnified detail shows the chip’s main control circuitry, including the startup electronics; the controller that determines whether to charge the battery, power a device, or both; and the array of switches that control current flow to an external inductor coil. This active area measures just 2.2 millimeters by 1.1 millimeters. (Source: MIT)

All of those operations also share a single inductor — the chip’s main electrical component — which saves on circuit board space but increases the circuit complexity even further. Nonetheless, the chip’s power consumption remains low.

“We still want to have battery-charging capability, and we still want to provide a regulated output voltage,” says Dina Reda El-Damak, an MIT graduate student in electrical engineering and computer science. “We need to regulate the input to extract the maximum power, and we really want to do all these tasks with inductor sharing and see which operational mode is the best. And we want to do it without compromising the performance, at very limited input power levels — 10 nanowatts to 1 microwatt — for the Internet of Things.”

The prototype chip was manufactured through TSMC’s University Shuttle Program.

Harvesting energy from rolling tires

A group of University of Wisconsin-Madison engineers and a collaborator from China developed a nanogenerator that harvests energy from a car’s rolling tire friction.

The nanogenerator relies on an electrode integrated into a segment of the tire. When this part of the tire surface comes into contact with the ground, the friction between those two surfaces ultimately produces an electrical charge, a type of contact electrification known as the triboelectric effect.

“The friction between the tire and the ground consumes about 10 percent of a vehicle’s fuel,” said Xudong Wang, an associate professor of materials science and engineering at UW-Madison. “That energy is wasted. So if we can convert that energy, it could give us very good improvement in fuel efficiency.”

During initial trials, Wang and his colleagues used a toy car with LED lights to demonstrate the concept. They attached an electrode to the wheels of the car, and as it rolled across the ground, the LED lights flashed on and off. The movement of electrons caused by friction was able to generate enough energy to power the lights, supporting the idea that energy lost to friction can actually be collected and reused.

The researchers also determined that the amount of energy harnessed is directly related to the weight of a car, as well as its speed. Therefore the amount of energy saved can vary depending on the vehicle-but Wang estimates about a 10-percent increase in the average vehicle’s gas mileage given 50-percent friction energy conversion efficiency.

Battery draws power from light

As a new means to harness light energy, a research group at the Indian Institute of Science Education and Research (IISER) in Pune, India developed a battery that can produce power from a light source. This development could potentially allow solar energy to be stored directly in a battery along with paving way to generate a sustainable photo battery.

The team developed a battery with a titanium nitride photoanode that is highly stable and safer than conventional options. Under normal indoor lighting, it discharged electric current and recharged within 30 seconds without an external power source. The photo battery worked for more than 100 cycles and could power an LED. Although not yet strong enough to run commercially available devices, the researchers say their design is a promising first step toward a more sustainable and safer battery technology.

The battery is able to run a small fan or an LED (Source: Dr. Musthafa’s lab, IISER, Pune)

Talking about the significance of this new design, Dr. Musthafa Ottakam Thotiyl of IISER said “In the state-of-the-art solar cells, a battery is always required to store solar power and that makes the system heavy and bulky. The present system can harvest and store solar energy in the same device thus integrating solar cell and battery functionalities. This way it is more economical and practical.”